Had a PS fluid leak from the two halves of the pump. Bought a seal kit and completely rebuilt the pump including new bearing and oil shaft seal. I am 100% confident that it is assembled correctly as I have referred to an array of youtube videos as well as subaru forums and even a honda resource (same pump).

With the pump installed I fill the pump with some oil as i turn the pulley before connecting the pressure and vacuum lines. I then completely fill the reservoir and bleed the system with lock to lock (engine off). After no more bubbles come to the surface I try starting the car and there isn't the usual issue of an air noise but instead there is massive shudder in the steering wheel that exists from lock to lock (front wheels are off the ground).

I'm thinking that the pump is struggling to maintain pressure to the rack?

With the car off you can hear some squishing sounds when turning the wheel that are coming from down near the rack.

Is the problem most likely vacuum hose, pulley belt or something else?

I had the same problem when I had by engine out and had to re-bleed the system. I assume it was air in the rest of the system (ie rack, etc) but it certainly gave me a fright. I actually turned the car off instantly because I thought it was engine!After a few good lock-to-locks (with decreasing shudder every time) the air was out of the system and it's been right as rain ever since.

man its sucking in air, the hose from the PS resovoir to the pump. If the hose is hard or the clamps aren't tightening properly itl suck in air. Even possibly if you can turn the hose whilst its clamped itll still suck in air. I work at Subaru and replaced a steering rack on a forester 06 and couldn't bleed it. Turns out it was sucking air in from the worn hose and clamps were completely worn.